I think it was as usual with BR done as cheaply as possible with only regard to operational use. Nobody would have imagined EMUs using P1 so all it was good for was as a shunt use for electric locos. Sticking up an 'Electric Trains Stop Here' sign was an easy way out.
To my untrained eye, no obvious 'easy' place to terminate/tension the wire beyond the buffer stops, whereas easier to plonk a mast alongside the track in some space once clear of the station building.